Hip prostheses, or artificial hips as they are sometimes known, have been in existence for many years, long enough to have accumulated a track record evidencing the strengths and weaknesses of the systems and their methods of implantation. In a total hip system, these prosthetic joints each consist of a substantially hemispherical acetabular cup adapted to be fixed into the acetabulum of the pelvis and a mating metallic ball which is carried by the proximal end of a long rigid femoral stem imbedded in the medullary canal of the proximal femur. Roughly sixty percent of these femoral stems are fixed in place by cement.
The number one complication of cemented total hip replacements is post-operative loosening of the components. Not only is this the most common complication, it rapidly increases with time, and in certain instances can be among the most severe complications following this kind of surgery. As an example of this severity, an increasing number of patients require a girdlestone procedure because of the severity of the failure and the massive bone destruction secondary to non-septic component loosening. Research figures indicate a 4% incidence of x-ray evidence of loose femoral components after two years. The same group of patients, when followed four to seven years later, evidenced a failure of cement fixation of up to 24%. Of special note was the incidence of 50% loose femoral components in one subset, those patients who had a small femoral component in a large canal.
Other figures show the failure on the femoral side of the prosthesis was 19.5% in cases followed three years post-operatively and this rose in a five to seven -year post-operative period to 42%. With the passage of time, the number of patients disabled with pain secondary to this loosening also rises, leading to a progressively increasing clinical failure rate.
The adequate securing of the acetabular component within the acetabulum has been the thrust of much research and development, but is not the subject of this invention and will not be discussed herein.
While a number of alternatives have been proposed to replace conventional cementing, none have yet proven to be a satisfactory solution. Better cements have not solved the problem nor is bony ingrowth sufficiently established to be widely adopted as an alternative in all cases. The surface replacement operation avoids the need to cement in a femoral stem, but the rapidly rising rate of early failure of surface replacement units makes the use of this approach limited.
For most surgeons, the optimum solution to this dilemma of loosening is to try to improve the traditional cementing techniques with such concepts as obtaining better fixation by virtue of improved intrusion of the cement into the trebecular bone by confining the cement in a specified space while increasing the intrusion pressure. To accomplish these ends, cement guns and compactor apparatus have been introduced, as well as the method of inserting a medullary plug at the base or distal end of the femoral stem, prior to cement injection, to restrict the flow of the cement to the space around the length of the stem. Yet another method sought to improve the cement fixation of the components is to centrifuge the cement prior to injection to expel entrained air bubbles.
In accordance with these state of the art methods and objectives, the stems are encased over their entire length in cement in order to provide a surrounding fixation casement between the stem and the femoral bone. Eliminating voids and bubbles in the cement maximizes the strength of the cement by adding to its homogeneity. Elaborate surfaces and metal finishes for the stem have also been designed to encourage bonding between the cement and the metal stem and, as mentioned earlier, the cement itself has undergone improvement to increase its penetration of the interstices of the trabecular bone.
Notwithstanding these efforts, however, loosening of the stem is a continuing problem after the prosthesis has been in place for a number of years, requiring revision surgery or other methods to alleviate the patient's pain and disability.
The most common area of failure and loosening of the femoral stem is in the proximal femur. While the cement fills the space between the metal stem of the prosthesis and the bone at the time of implantation, and in most case creates a satisfactory fixation, after time the bone itself becomes subject to osteoporosis and thinning which is followed by fracture of the cement mantle surrounding the stem, allowing the stem to toggle about its lower end as a pivot point. The cause of the osteoporosis is significant to the present invention. Bone, like living muscle tissue, develops and remains strong and healthy with use. In the case of bones like the femur, the accommodation of stress keeps the bone structure healthy.
Until this invention, it has been thought necessary to cement the entire length of the femoral stem to insure its adequate stability for the long femoral stem. When that is done, however, the compressive and other stresses created in and through the stem when the leg is used are transferred to the femoral bone near the distal end of the prosthetic stem, not uniformly along its length. The fixation created between the cement and the portion of the bone surrounding the lower portion of the stem transfers the forces developed through the ball joint to the lower portion of the femur and by-passes its proximal end --that is, there is stress shielding of the proximal femoral bone. Over a period of time, that portion of the bone which is stress-shielded is subject to the deterioration of osteoporosis and begins to atrophy and thin; the cement mantle proximally fractures, usually radially of the stem, allowing the cement mantle to spread open, bringing on unacceptable toggle movement of the stem within the bone. It has been discovered as a fundamental part of this invention that cementing the entire length of the femoral stem is not only unnecessary, but is counter-productive to the objective of maintaining fixation of the stem within the femur over long periods of time. It is further apparent through the teaching of this invention that cementing only the proximal portion of the femoral stem, from the lower metaphysis to the plane of the transverse cut of the femoral neck osteotomy, will eliminate osteoporosis of the bone by transferring the stresses to the proximal femur and not shielding them therefrom.
It is therefore the principle object of the present invention to provide an apparatus and method for restricting the length of the cement mantle surrounding the femoral prosthesis stem so that the lower end of the stem will not be cemented, thereby creating a system for transferring the dynamic forces generated in the upper portion of the stem to the proximal femur and thus eliminating stress shielding of the proximal femur and the consequent deterioration of the bone in that area.
Within that overall objective, it is a further objective of the invention to provide a plug which is inserted in the specially prepared medullary canal, prior to the injection of the cement, which will prevent the flow of cement past the plug toward the distal end of the femoral stem.
A still further object of the invention is to provide a novel method of implanting a femoral stem which eliminates the need for large quantities of cement which, in a large-boned person, may require, under state of the art techniques, a refilling of the cement gun during a time-critical portion of the surgery.
Other and further objects, features and advantages of the invention will become apparent from a reading of the detailed description of the invention which follows.